The present invention relates to fuels for homogeneous charge compression ignition engines, more specifically to those having an excellent ignitability and capable of enhancing the engine output and widening the engine speed range as much as possible so as to improve the engine thermal efficiency.
Nowadays, two types of engines have been widely used, one of which is a spark ignition gasoline engine and the other of which is a compression ignition diesel engine.
For the spark ignition gasoline engine, fuel is injected into the intake port or the combustion chamber, and premixed gas of air fuel mixture is formed. Then the premixed gas is ignited by a spark plug and combusted. The fuel is required to have high vaporization and low auto-ignitability characteristics. Since the spark ignition gasoline engine emits nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide, a three-way catalyst has been widely used for purifying these emissions. However, an exhaust gas purification system such as a three-way catalyst is only applicable to a range where the air-fuel ratio is in a very narrow range of stoichiometric air-fuel ratio and it is the causes of low thermal efficiency and poor fuel consumption comparing with the compression ignition diesel engine.
For the diesel engine, a diesel fuel is directly injected into the cylinder and mixed with the air during compression stroke. The air-fuel mixture is auto-ignited by increasing the temperature and pressure by piston compression. The diesel fuel is required to have high ignitability characteristics. The compression auto-ignition diesel engine is excellent in fuel consumption and thermal efficiency but has disadvantages of NOx and soot emissions caused by the heterogeneous air fuel mixture. Furthermore, severe control of an after treatment system such as an oxidation catalyst, NOx trap, a diesel particulate filter or an SCR system is required to reduce NOx and soot to meet political regulations.
Therefore, the conventional spark ignition gasoline engine can purify the exhaust gas to a certain extent but has problems regarding fuel consumption and thermal efficiency. On the contrary, the diesel engine is excellent in fuel consumption and has high thermal efficiency, but it has problems of emission of NOx. Therefore, a homogeneous charge compression ignition engine has been studied to achieve low NOx exhaust gas, excellent fuel consumption and high thermal efficiency.
For the homogeneous charge compression ignition engine, the fuel is injected into the intake port or combustion chamber at an injection pressure of 20 MPa or lower, which is extremely lower than the diesel engine and the fuel injection is completed at a crank angle of 60 degrees before the top dead center so that a premixed air-fuel mixture is combusted by auto-ignition but not by spark ignition. The homogeneous charge compression ignition engine takes a longer period to prepare a well-mixed air-fuel mixture in the cylinder, comparing with the diesel engine. Therefore, for the homogeneous charge compression ignition engine, a high temperature combustion region, the temperature of which is higher than 2200K, is not locally formed in the cylinder and this is the cause of low NOx emission characteristics (less than 10 ppm by mass) without a reduction catalyst. The thermal efficiency and fuel consumption of the homogeneous charge compression ignition engine are equivalent to those of the diesel engine.
Various fuels for the homogeneous charge compression auto-ignition combustion engine have been proposed, focusing on various indices such as ignitability, volatility, cetane number and octane number (for example, see Patent Documents 1 to 13 below) However, more optimum and suitable fuels for homogeneous charge compression ignition have been demanded from the point of engine performances.    Patent Document 1: Japanese Patent Laid-Open Publication No. 2004-919657    Patent Document 2: Japanese Patent Laid-Open Publication No. 2004-919658    Patent Document 3: Japanese Patent Laid-Open Publication No. 2004-919659    Patent Document 4: Japanese Patent Laid-Open Publication No. 2004-919660    Patent Document 5: Japanese Patent Laid-Open Publication No. 2004-919661    Patent Document 6: Japanese Patent Laid-Open Publication No. 2004-919662    Patent Document 7: Japanese Patent Laid-Open Publication No. 2004-919663    Patent Document 8: Japanese Patent Laid-Open Publication No. 2004-919664    Patent Document 9: Japanese Patent Laid-Open Publication No. 2004-919665    Patent Document 10: Japanese Patent Laid-Open Publication No. 2004-919666    Patent Document 11: Japanese Patent Laid-Open Publication No. 2004-919667    Patent Document 12: Japanese Patent Laid-Open Publication No. 2004-919668    Patent Document 13: Japanese Patent Laid-Open Publication No. 2004-315604